Locations within books have traditionally been indicated by page and line numbers. However, the concept of a “page” is not as meaningful in the context of electronic books, in which page divisions may be calculated dynamically, as a function of page and font size. Different book reader devices have different display sizes, and even within a particular device, a user may change viewing options to result in different effective page sizes.
Because of this, locations within electronic books are often specified in terms of an index that relates to a smaller unit of measure, such as by a byte/character index or a word index. For example, a particular word might be specified in terms of its starting and ending character indexes, relative to the beginning of the electronic book.
However, a further complication arises when dealing with multiple versions of an electronic book. When publishing electronically, revisions are very easy to implement, and publishers often submit new versions to correct relatively minor things such as typographical errors, as well as to make more significant revisions. More specifically, different versions may arise because of updates from the original publisher, because different publishers might submit versions of the same book, because of differences in electronic formats, because of differences in media (such as electronic vs. audio), and so forth.
Even small changes between versions can disrupt a position indexing scheme, making it difficult to correlate locations across versions.